An actuator is a mechanical, pneumatic, hydraulic, or electrical device that moves a body from an initial position to a subsequent position in response to a signal. Actuators are used in numerous applications. For instance, an actuator may be used as a switch that closes a circuit when a conductive body of the actuator moves from an initial position to a subsequent position. An actuator also may be used as a valve that shuts off fluid flow in a channel when a valve body of the actuator moves from an initial position to a subsequent position.
Pyrotechnically powered actuators have been used in missiles, launch vehicles, spacecraft, and many other applications. In this context, actuators can be used for igniting, moving, separating or activating various elements. Generally, pyrotechnic actuators are fired (triggered) by electro-pyrotechnic components in which at least one phase involves the rapid decomposition of pyrotechnic substances at high pressure and temperature. These devices typically use pressure cartridges or explosive charges to provide the high pressure, high temperature gases to move a piston to a desired stroke.
FIG. 7 presents a cross-sectional side view of a known pyrotechnic actuator 700 having a piston assembly. Actuator 700 includes a housing body 708 that receives a piston 710 and an initiator 706, which is an igniting system. Piston 710 is held in place within housing body 708 by a shear pin 712 that protrudes through piston 710 and housing body 708. Initiator 706 includes a cover 720 having holes through which leads 702a and 702b extend and an inner surface upon which a wire bridge element (not shown) is attached such that it contacts leads 702a and 702b. An end of each of leads 702a and 702b is attached to a power source (not shown). Initiator 706 is filled with pyrotechnic material. During initiation the power source is energized, which causes the leads to trigger the wire bridge element, igniting the pyrotechnic material. This ignition causes the rapid expansion of gas, which results in extremely high pressure within housing body 708.
O-ring 704 provides a tight seal around a head 716 of the piston 710 to maintain pressure in housing body 708 between head 716 and cover 720 after initiation. Pressure must be maintained behind head 716 so that high pressure produced by initiation forces head 716 to move piston 710 quickly and with enough force to break shear pin 712. Dotted lines 714 illustrate the stroke provided by piston 710 upon initiation. The movement of piston 710 is confined to the distance head 716 can move within housing body 708.
In addition to o-ring 704, actuator 700 requires close tolerances, allowing only a small difference between maximum and minimum limits of each dimension, so as to create a seal. Tight seals are important because high pressures can cause blow-by, contamination, and leakage, which can cause potentially catastrophic results.
Another type of actuator uses expanding bellows that move from an initial, shorter position to a final, expanded position. Typically, bellows have been made of brass or gilding metal, which tend to rupture under internal or external pressure under 2,000 psi. Conventional bellows tend to deform in multiple directions as a result of high internal pressure, which causes an irregular stroke.
Referring again to FIG. 7, in a conventional pyrotechnically powered actuator, leads 702a and 702b supply a relatively large current for triggering the actuator. A typical pyrotechnically powered actuator requires a minimum of 3.5 amps of power for at least 10 ms to function reliably. The bridge is generally large and requires a relatively high threshold current to be tolerant of stray currents and voltages throughout the system that otherwise could cause false triggers. In this manner, the bridge dissipates these currents. As a result, initiators for conventional pyrotechnic actuators typically are large and heavy. Complex systems may include many initiators, which often require large and heavy cables, controllers and batteries. The cables used are typically at least as large as 18 gauge to be sufficient to carry large transient currents of one to five amps during firing. In the aggregate, the large number of high-power shielded cables required for the branching configuration of actuators are heavy and occupy significant volume, resulting in weight and packaging difficulties within an aircraft, spacecraft, missile, launch vehicle or other application where weight and space are at a premium. Accordingly, this increase in pyrotechnic system weight and volume, coupled with the pressure limits discussed above, presents difficulties may require significant engineering time to solve.